An inverter apparatus is provided with a power device circuit such as a bridge circuit. The bridge circuit includes a plurality of switching elements. As the switching elements, switching elements of a voltage control type (hereinafter, also referred to as “voltage control type switching elements”), such as insulated gate bipolar transistors (abbreviation: IGBTs) and metal-oxide-semiconductor field effect transistors (abbreviation: MOSFETs), are commonly used.
A circuit that controls a gate voltage of a voltage control type switching element is referred to as a switching element drive circuit (hereinafter, also simply referred to as a “drive circuit”). The drive circuit performs switching of the voltage control type switching element.
As disclosed in Patent Document 1, for example, the drive circuit outputs a power supply voltage of the drive circuit at turn-on of the voltage control type switching element (hereinafter, also referred to as a “switching element”), and outputs a voltage of 0 V or a negative voltage at turn-off of the switching element. The drive circuit includes a gate resistor, and is connected to the gate of the switching element via the gate resistor.
An increase in resistance of the gate resistor decreases rising and falling speeds of a gate voltage that is a voltage applied to the gate of the switching element, leading to a decrease in switching speed of the switching element. On the other hand, a decrease in resistance of the gate resistor increases the rising and falling speeds of the gate voltage, leading to an increase in switching speed of the switching element. As such, the drive circuit can control the switching speed of the switching element by use of the gate resistor.
Drive circuits to improve switching characteristics of switching elements and to suppress short circuit current are disclosed in Patent Documents 2-7, for example. For example, Patent Document 2 discloses a drive circuit in which an on-side circuit that performs a turn-on operation of a switching element and an off-side circuit that performs a turn-off operation of the switching element are each provided with two gate drive elements and a delay circuit, and the delay circuit switches between the gate drive elements, for the purpose of reducing a switching loss of the switching element. In the following description, the gate drive elements of the on-side circuit are also referred to as “on-side gate drive elements”, and the gate drive elements of the off-side circuit are also referred to as “off-side gate drive elements”.
When the switching element is turned on, the drive circuit disclosed in Patent Document 2 first gradually increases a gate voltage via a first on-side gate drive element and a gate resistor of the on-side circuit. The drive circuit then provides a voltage applied to a second on-side gate drive element to the gate of the switching element with some delay caused by the above-mentioned delay circuit to increase the gate voltage to a voltage in a steady state.
When the switching element is turned off, the drive circuit disclosed in Patent Document 2 first gradually decreases the gate voltage via a first off-side gate drive element and a gate resistor of the off-side circuit. The drive circuit then decreases the gate voltage to ground potential with some delay by use of the above-mentioned delay circuit and a second off-side gate drive element.
That is to say, the drive circuit disclosed in Patent Document 2 increases the gate voltage of the switching element in two stages by switching between a plurality of gate drive elements over time when the switching element is turned on, and decreases the gate voltage of the switching element in two stages by switching between a plurality of gate drive elements over time when the switching element is turned off. The gate voltage is thereby gradually changed.
Radiation noise is reduced by gradually changing the gate voltage as described above. Furthermore, a switching loss is reduced by suppressing a turn-on time that is a time required for the switching element to switch from an off state to an on state, and a turn-off time that is a time required for the switching element to switch from the on state to the off state.
Patent Document 7 discloses a drive circuit in which a power supply voltage or an input voltage is varied between an on-side circuit and an off-side circuit to achieve, in addition to reduction of a switching loss, suppression of arm short circuit current and load short circuit current in a case where switching elements are bridge-connected. The drive circuit disclosed in Patent Document 7 turns on the switching element at a relatively low voltage by use of the on-side circuit, thereby suppressing the arm short circuit current and the load short circuit current.